Clear evidence exists that CD8 T cell responses to the circumsporozoite protein of the rodent Plasmodia (P. berghei or P. yoelii) can stop the infection at the liver stage in BALB/c mice, thus preventing the symptomatic blood stage of malaria. Our laboratory has spent considerable effort over the last decade developing approaches to stimulate and analyze CD8 T cell memory responses after infection and vaccination. Our initial goal in entering the malaria field was not to develop a vaccine, but instead to devise a model system that would permit us to study the immunological mechanisms resulting in protective immunity mediated by memory CD8 T cells. As shown in the preliminary data of this proposal and a recent publication in PNAS1, our laboratory has developed an immunization strategy to generate P. berghei circumsporozoite (CS)-epitope- specific memory CD8 T cells that provides essentially life-long protection of BALB/c mice against multiple challenges with Plasmodium sporozoites (spz). We used this model to determine that CS-specific memory CD8 T cells exceeding a large (>20% of CD8 T cells), but definable frequency (>1% of all PBL) were required for sterilizing immunity. Our long-term goal is to exploit this quantitative model system to understand the basic immunological mechanisms that result in memory CD8 T cell-mediated protective immunity against the liver stage of Plasmodium infection, information that could be critical for the rational design of efficacious vaccines against malaria. We will address this long-term goal through the following specific aims: Aim 1. Determine if the large threshold of CS-specific CD8 T cells required for protection of BALB/c mice against P. berghei infection is generalizable to other Plasmodium species/epitopes or other mouse strains. Aim 2. Determine how CS-specific memory CD8 T cells protect against liver stage Plasmodium infection. Aim 3. Determine if and how recruitment of other immune effector cells and pathways reduces the threshold required for CS-specific CD8 T cells to provide sterilizing immunity to Plasmodium challenge. Aim 4. Determine if recurring infections with unrelated pathogens result in attrition of CS-specific memory CD8 T cells and compromise sterilizing immunity. PUBLIC HEALTH RELEVANCE: Much emphasis in the field of subunit vaccines for malaria has been placed on improving the vector systems for antigen-delivery and these empirical approaches are likely to improve the existing delivery platforms and potentially reveal new platforms. However, despite the more than 40 years of research in malaria vaccines, we still know very little about the specific immune mechanisms required for sterilizing immunity to Plasmodium infection. Addressing this knowledge gap would aid in the rational design of malaria vaccines and is the goal of this proposal.